BV-1

Cooperative Extension Service
Purdue University
West Lafayette, IN 47907

Matching Multiple Ventilation Fans

A. J. Heber,
Department of Agricultural and Biological Engineering

Fans are often used to ventilate agricultural buildings. Adjoining
rooms with connecting air paths through open doors or manure channels
can operate as a single room with a common negative pressure. The
pressure capability or strength of multiple ventilation fans must be
carefully matched to allow all fans to operate efficiently. Under
ventilation, back drafting, and poor air distribution may result when
a fan exhausts air from a room in parallel with a stronger fan,
especially in tightly constructed buildings. The fan mismatch problem
is not immediately obvious to the casual observer, but may result in
depressed animal health and performance in livestock buildings,
reduced human comfort in residences, contaminant exposure of products
in food plants, and lower air quality in all buildings. But there are
ways to check whether the problem exists in ventilated buildings. The
following cases illustrate a few common problems and solutions.

Variable-Speed Fans in Livestock Buildings

It is not uncommon to see a variable-speed fan in the second
ventilation stage while the first-stage fan is running at full
speed. This scenario should be avoided. One fan controller that
implements the correct strategy will be described later.

To visualize the consequences of such a mismatch, imagine two
straws in an empty soda cup with a well sealed lid. Now, imagine a man
and small child attempting to suck air from the cup at the same
time. The child's sucking power is no match for the adult's. Neither
is a variable-speed fan running at low speed a match to one running
at full speed.

Here is what happens when variable-speed fans are mismatched in a
livestock building:

1.Variable-speed fans likely operate at reduced air flow and
efficiency when stronger fans are exhausting from the same room. In
the worst case, back drafting occurs and the fan acts as a drafty
inlet. It isn't very good because it is usually in a poor location and
the incoming air speed isn't high enough to keep cold air off the
pigs.

2.The variable-speed fan becomes extremely vulnerable to wind
pressure. Moderate winds (6 to 12 mph) can slow and even reverse
variable-speed fans at low speeds even without back pressure from
other fans (Heber et al., 1993).

3.Variable-speed fans get a bad rap because of poor performance
resulting from fan mismatches. This should be a concern for fan
companies and rural electric suppliers.

Some solutions to these problems include:

1.Operate a variable-speed fan with another fan that is identical and
running at the same speed. After the first-stage, variable-speed fan
is at full speed and the room needs more air, decrease its speed to
50% airflow and increase the speed of the second-stage variable-speed
fan starting it also from 50% airflow. Both fans then always operate
at about the same speed. At least one electronic controller on the
market is capable of carrying out this strategy (Heber, 1991).

2.Use a variable-speed fan as a winter, minimum ventilation fan only,
unless the above solution can be implemented.

3.If a variable-speed fan must operate with a full-speed fan, raise
its minimum speed setting to at least 50% to reduce back drafting and
performance degradation.

4.Avoid using variable-speed fans in adjoining rooms that have
unavoidable air paths between the rooms. Single-speed fans are more
likely to match the pressure created by other single-speed fans.

Multiple Single-Speed Fans in Livestock Buildings

The words "weak" and "strong" in this bulletin refer to pressure
capability, not airflow. A fan that has three, low-angle blades on a
small hub would probably be "weak" compared to an impeller with six,
high-angle blades on a large hub. A high-speed fan rotating at 3200
rpm would likely be "stronger" than a fan rotating at 1600 rpm.
Likewise, a two-speed fan is weaker at the lower speed.

1.Weak fans operate at reduced airflows and reduced efficiencies when
stronger fans are running (and they will at higher outside
temperatures). This condition could exist for most of the non-heating
season.

3.Air may move from a room with a weak fan to an adjoining room with a
strong fan. Air movement between rooms enhances airborne transfer of
pathogens and tends to generally mess up the ventilation in the room
with the strong fan.

4. Ventilation airflow may be reduced by 5 to 20% for most of the
year.

Here are some solutions to the above problems:

1. Turn weaker, first-stage fans off while operating higher pressure
fans. The higher pressure fans then need to be larger to compensate
for the capacity intended from the weaker fan.

2. Seal air paths between adjoining rooms so each room operates at its
own negative static pressure.

3. Install fans that have the same design pressure. All fans should
operate at about the same speed. One might have one or more single
speed fans in the wall that operate at about 3300 rpm and other fans
that operate at about 1650 rpm. This should always be avoided because
it is likely the fans are mismatched unless the high-speed fan is
connected to pit ducts.

Figure 1. Exhaust fans and air inlets in a mechanically ventilated
building.

When adjoining rooms have connecting air paths through open doors or
manure channels, they operate as a single room with a common negative
pressure. If that is the case, then fan matching between rooms becomes
an issue.

An analogous situation occurs in a residence with a small fan in the
bathroom and a large, whole-house ventilation fan in another room or a
hallway. When both fans operate, the small bathroom fan ends up
operating at reduced efficiency and reduced or negative airflow. As a
result, odors and moisture exhaust more slowly or enter other rooms in
the house. Implementing solution #2 in the list above would be to shut
the bathroom door (unless the door is the only air inlet to the
bathroom).

Food Plant Ventilation Systems

A food manufacturing plant may have several types and sizes of fans
exhausting from the same room or adjoining rooms. High pressure power
ventilators, tube-axial fans, centrifugal fans, room heaters, and
vacuum systems sometimes operate in parallel with low pressure
propeller fans.

Fan mismatches like this may result in the following problems in food
plants:

3.Airflow from areas with contamination sources to clean areas such as
meat packaging may be reversed. This may increase the risk of
cross-contamination.

The fan mismatches can be eliminated by:

1.Replacing weak fans with stronger or higher pressure fans,

2.Replacing strong fans with weaker fans, and

3.Providing better air seals between rooms to prevent cross flow.

Figure 2. Top view of three exhaust ventilation fans operating in
parallel.

Technical Explanation of Fan Mismatch Problems

Static Pressure and Air Flow of a Ventilation Fan

Static pressure developed by a ventilation fan depends on the
resistance to airflow offered by the ventilation inlets. The
resistance as indicated by pressure drop through the ventilation
inlets increases with airflow rate and is represented by the system
curve (Figure 3). Notice that doubling the airflow generally increases
the pressure drop by four times at any given inlet opening.
Increasing the inlet area, e.g., opening inlet baffles, moves the
system curve downward to lower pressures.

Figure 3. Characteristic curve and ventilation efficiency ratio
(VER) of a 16-in. variable speed fan operating at several control
voltages. The lower system curve is created by increasing air inlet
openings. (iwg=inches of water gauge pressure and cfm/W is cfm
of air flow per watt of electrical power). Source of data: Kansas
State University (Heber et al., 1989).

At a given voltage, a fan has its own characteristic curve that gives
the airflow it produces over a range of static pressure. A fan always
operates somewhere on its characteristic curve. Voltage adjustments by
the fan controller create new characteristic curves. The relationship
between pressure and airflow is rarely a straight line and the curve
sometimes has a very distinct stalling region, especially with
propeller fans, Figures 3, 4 and 5. Stalling is described later.

The free air point, where there is no pressure developed by the fan,
is located at the low end of the pressure range. A fan develops its
maximum air flow at free air (Figure 3). Free air is experienced by
circulation fans and by exhaust fans when all wall curtains are wide
open.

The cutoff point, where a fan may be rotating but not moving out
any air, is at the upper end of the pressure range (Figure 3). Cutoff
occurs when a fan exhausts from a space that has no air inlets or when
a strong wind induces just enough pressure on the fan blades to cause
zero air flow through it. A fan normally operates somewhere between
the free air and cutoff pressures and preferably at the point of
highest impeller efficiency.

Impeller efficiency is highest at pressures between free air and
stalling. More interesting, however, is the fan's ventilation
efficiency ratio (VER) which is air delivered per unit of electric
power. VER curves are shown in Figure 3 for a 16-in. fan at five
control voltages. VER is highest at free air and drops to zero as
pressure goes up to cutoff. Therefore, excessive fan pressure means
decreased efficiency, wasted electric energy, and insufficient
ventilation.

Figure 4. Characteristic curve of a 24-in. variable speed fan
operating at several control voltages. Source of data: Kansas State
University (Heber et al., 1989).

The operating point of a fan is dictated by the intersection of
the system curve and the fan characteristic curve. Some operating
points are shown by large dots in Figures 3 to 8. For example, the
94-in variable speed fan delivers about 4100 cfm at full speed (at 115
V) at operating point A in Figure 4.

Fan stalling does not mean that the fan quits turning. Rather,
stalling occurs when the fan suddenly experiences a greater drop in
air flow with additional pressure. As Figure 3 shows, a small pressure
increase, (0.26 to 0.36 in. of water gauge (iwg), for example) can
move the fan from operating near its rated pressure and airflow to the
stalling region causing airflow and VER to drop by up to 50%.

Figure 5. Characteristic curve of a 12-in. variable speed fan
operating at several control voltages. Source of data: Kansas State
University (Heber et al., 1989).

Further pressure increases (0.36 to 0.60 iwg) because of wind,
insufficient inlets, or fan competition will reduce airflow and
efficiency further. It is important to point out that fan
characteristics vary a lot from one model to another. Stalling also
varies in severity, It depends on blade and housing design and the
effect of guards, shutters, and other obstructions to airflow.

Airflow and efficiency reductions often go unnoticed in buildings
because fan speed is about the same and the fans appear to be
operating normally, The following paragraphs explain how competition
from mismatched fans causes a fan to depart from its design point and
how such departures can be avoided.

How Multiple Fans Move Air

When more than one fan exhausts air from a room (Figures 1 and 2),
the fans are said to operate in parallel. Because they are operating
in parallel from a relatively large space, all fans must operate at
the same static pressure. A combined characteristic curve for
multiple fans is determined bv adding the airfiows at each
pressure. For example, the characteristic curves of two 16-in. fans,
one operating at 55 V (or 45 V) and the other operating at 115 V, are
combined in Figure 6. Each fan operates at a common pressure as
dictated by the operating point on the combined curve. All the fans
should ideally have the same design pressure and the air inlet system
should allow the fans to operate at that design pressure. Pressure
mismatches become a problem when the design pressures of multiple fans
are significantly different.

Parallel fan mismatches are not serious at pressures at or near
free air. However, the design pressure of ventilated agricultural
buildings is generally about 0.10 to 0.125 iwg and manufacturers
typically design them so the highest propeller efficiency occurs at a
pressure of 0.125 iwg. Actual design pressure may differ from 0.125
iwg depending on the quality of the final product.

At lower voltages, variable-speed fans create new characteristic
curves that have lower design and cutoff pressures (Figures 3, 4 and
5). Thus, a variable-speed fan operating at its lowest speed has much
lower design and cutoff pressures than an identical fan operating at
full speed. For example, the 16-in. fan in Figure 3 has a cutoff
pressure of 0.9 iwg at 115 V but less than 0.1 iwg at 45 V or low
speed. That means the 16-in. fan at low speed cannot move any airflow
if it is operating against an external pressure greater than 0.10 iwg.

Figure 6. Combined characteristic curves of two identical fans, one
operating at low voltage and one operating at full voltage.

Common Fan Mismatches

The following cases illustrate some common fan mismatches that can
occur in livestock buildings. The three commercial fans used in the
examples were designed for livestock buildings and were thoroughly
tested in the laboratory. Other fans will have different
characteristics but the general concepts illustrated still apply.

Case 1. Two Identical Fans, One Operating at Low Speed and One
Operating at Full Speed

Figure 6 shows two identical 16-in fans, one fan running at low
speed and one at full speed. This case is very common, especially
with the advent of integrated controllers that are able to stage more
than one variable-speed fan. With the slow fan operating at 45 V, the
system curve intersects the combined curve (Point B) at about 0.09 iwg
and 3,000 cfm . Since the cutoff pressure of the slow fan at 45 V is
only 0.07 iwg and less than the operating pressure of 0.09 iwg, it
cannot exhaust any air. In fact, air will flow in reverse through the
slow fan so that it becomes an inlet for the full speed fan!

Of course, reverse flow will be blocked by back draft
shutters. However, back draft shutters do not always seal perfectly
thus allowing some air to flow back through the fan. All this can
happen while the fan impeller rotates in the positive direction!
Operating by itself, the slow fan moves 1,050 cfm (point D). Turning
the full-speed fan on again causes reversal of flow through the slow
fan (see dashed line from point B).

Since the building will be under-ventilated with the slow fan at
only 45 V indoor temperature will gradually rise causing fan control
voltage to increase. When it reaches 55 V we get the new operating
point A, which is now at a pressure less than the cutoff pressure of
the slow fan (Figure 6). The slow fan now operates at point E on its
individual characteristic curve. Since the fan is stalled at point E,
the slow fan's efficiency and airflow are seriously compromised.

When the high-speed fan is turned off, the operating point for the
slow fan at 55 V changes from point F (0.125 iwg and 600 cfm) to point
C (0.05 iwg and 1,650 cfm), as shown in Figure 6. Therefore,
operating the full-speed fan causes the flow rate of the slow fan to
drop by 64%. Checking Figure 3, the VER drops from 8.5 to 3.0 cfm/W.
Blowing 600 cfm at 3.0 cfm/W as compared to 8.5 cfm/W costs
$9.53/month at 10 cents/kWh.

Also, the slow fan becomes extremely vulnerable to further
airflow reductions, complete shutdown, and even speed reversal from
wind induced pressure. At 0.125 iwg, the 0.04 iwg of additional static
pressure needed to completely shut down its airflow can be created by
a head-on 9 mph wind.

Electronic controllers introduced to the livestock industry in
recent years are often used to stage more than one variable-speed
fan. After the first stage fan is brought to full speed, rising
temperatures bring a second fan on at low speed, thus creating an
unhealthy pressure mismatch. The mismatches generally go unnoticed
because:

1.Fans are rotating in a positive direction,

2.Air and/or its rate of flow are not readily
visible, and

3.The controller slowly but eventually
increases fan speed until proper ventilation
is achieved.

The minimum control voltage of the second fan should at least be
high enough to prevent back drafting that occurred with the slow fan
at 45 V (Figure 6), and preferably high enough to prevent stalling.

Figure 7. Combined characteristic curve of a 24-in. fan operating at
low voltage and a 16-in. fan operating at full voltage.

Case 2: Large Pan at Low Speed and Small Pan at Full Speed

Ventilation systems in livestock facilities often use a small fan
for the first stage of ventilation and a larger fan for the second
stage. Sometimes, both fans are variable speed and the large fan is
started at low speed while the smaller fan is running full speed.

Figure 7 shows the characteristic curves of a 24-in. fan operating
at 40 and 50 V individually and in combination with a 16-in. fan
operating at full speed. At 40 V the combined operating point is 0.1
iwg (Point B) and 3,000 cfm. Amazingly, the 24-in. fan is completely
overpowered by the smaller 16-in. fan! This happens because the 0.1
iwg operating pressure for both fans is much larger than the cutoff
pressure of 0.065 iwg for the 24-in. fan. At 50 V the combined
operating point is 0.125 iwg and 3,250 cfm (Point A), but the 24-in.
fan is only contributing 350 cfm (Point E) and very inefficiently. By
itself, with the same system curve (inlets not readjusted), the
24-in. fan at 50 V would move 1,250 cfm or 3.5 times as much air.

Looking at the characteristic curves for the 12-in. and
24-in. fans shows that the 12-in. fan running at full speed can cause
flow reversal through the 24-in. fan at its lowest speed (control
voltage 40 V).

Therefore, avoid operating another fan at full speed, whatever its
size, with a variable speed fan at low speeds.

Case 3: Large and Small Pans Operating Full Speed

Fan mismatch problems are also possible when all fans operate at
full speed. Figure 8 shows how the performance of a 12-in. fan is
hampered when operating in parallel with a 24-in. fan. The stalling
pressure for this particular 12-in. fan is only 0.08 iwg (probably
low compared to most 12-in. fans). A typical operating point for both
fans running is 0.10 iwg and 4,600 cfm (Point A). The pressure
developed by both fans together forces the 12-in. fan to operate at
Point D on its characteristic curve, blowing only 375 cfm. Compare
this to Point C where the fan operating by itself moves 850 cfm!

Figure 8. Individual and combined characteristic curves of 24-in. and
12-in. fans operating at full voltage.

One might rightly argue that the inlets would be adjusted to
smaller openings thus moving the system curve upward. This might make
the 12-in. fan operate at Point E (0.05 iwg and 750 cfm). Even so,
the fan pressure mismatch still causes a 50% reduction in airflow
produced by the 12-in. fan.

This problem can be corrected by choosing a small fan with higher
pressure ratings. Another solution may be to shut the 12-in. fan off
when operating the 24-in. fan. The results of this strategy are shown
in Table 1.

a

b

Troubleshooting

Severe mismatching of parallel connected fans can be checked in
the field. If, while the multiple fans are operating, the back draft
shutters of a fan are shut or nearly shut, then there may be a
problem. Watch the shutters while turning off the other fan or
fans. If the shutters open up when turning the other fan or fans off,
then the fan is "weak" compared to the other fans. For fans without
shutters, use a vane anemometer or anything that would "blow in the
wind" to visualize the exit air velocity from the fan.

This procedure was conducted in a hog producer's building. The
shutters of this fan (Figure 9a) were wide open with the fan operating
at about 50% speed or so. An identical fan about 20 feet away in the
same room was turned on at full speed. The result? The shutters of
this fan slammed shut (Figure 9b). Turning the full speed fan off
caused the shutters to open up again.

The experiment was repeated with an outside door wide open. This
time, the other fan had no effect on this fan. Why not? Because
opening the door simulated a very loose building and a free air
condition. Fan mismatches are not a problem at free air.

If a fan is operating at a pressure less than the stalling
pressure, air will move outward, even near the hub. However, when a
fan is stalled, air is sucked back into the fan near the hub. As
pressure increases more, the circle of reverse flow grows. A strand of
an ostrich feather attached to a dowel rod works well for checking air
direction at the fan outlet. The size of the reverse flow region can
be readily measured. If the size changes when turning off other fans,
then the fan is suffering from a fan mismatch.

Summary

Ventilation control strategies in mechanically ventilated buildings
can be improved by avoiding fan mismatches. Mismatches occur when the
static pressure capabilities of parallel connected fans are not
compatible. Sometimes fixed speed fans can be mismatched depending on
their respective design pressures. The most serious mismatch occurs
when variable speed fans are operated at low speeds when other fans
are operated at high speeds. This bulletin described several cases of
fan mismatches and explained how the problems can be avoided.

Cooperative Extension work in Agriculture and Home Economics, State of
Indiana, Purdue University and U.S. Department of Agriculture
cooperating: H.A. Wadsworth, Director, West Lafayette, IN. Issued in
furtherance of the acts of May 8 and June 30, 1914. The Cooperative
Extension Service of Purdue University is an equal opportunity/equal
access institution.